An artist's impression of Alpheus and its moon
Date March 12, 2006
Discoverers Wright et al.
Detection method Radial velocity
Site Keck Observatory
Name & designations
Pronunciation /'ul•fē•yis/
Adjective Alphian
Planet numbers P170, HD 154345 P1, Hercules P4,
Tarandus P15, 2006 P6, 2006 Her-1,
2006 Tar-2
Star designations PH 134 b, P3 Herculis b,
P13 Tarandi b, HD 154345 b,
HIP 83389 b, Gliese 651 b,
SAO 46452 b
System Gliese 651
Constellation Hercules
Caelregio Tarandus
Right ascension 17h 02m 36.40s (255.651 68°)
Declination +47° 04' 54.8" (+47.081 88°)
Distance 18.587 pc (60.622 ly)
Orbital characteristics
Semimajor axis 4.359 529 AU (652.176 2 Gm)
Periastron 3.208 965 AU (480.054 4 Gm)
Apastron 5.510 092 AU (824.298 0 Gm)
Eccentricity 0.263 919 2
Orbital circumference 27.037 02 AU (4 044.680 Gm)
Orbital area 57.590 57 AU² (1 288 849.43 Gm²)
Orbital period 3 537.667 355 d (9.685 605 35 yr)
Avg. velocity 13.278 km/s (2.791 AU/yr)
Max. velocity 14.927 km/s (3.698 AU/yr)
Min. velocity 11.392 km/s (1.383 AU/yr)
Direction of orbit
relative to star's rotation
Inclination 87.452° to ecliptic
−1.042° to star's equator
−3.334° to invariable plane
Argument of periastron 68.103°
Longitude of ascending node 325.461°
Longitude of periastron 33.564°
Angular separation 225.713 mas
Observing the parent star
Mean angular star size 0.107 40° (6.444')
Max. angular star size 0.145 91° (8.754')
Min. angular star size 0.084 97° (5.098')
Mean star magnitude −22.984
Max. star magnitude −23.649
Min. star magnitude −22.475
Bulk characteristics
Mean radius 1.081 86 RJ (75.668 Mm)
Equatorial radius 1.082 92 EJ (77.380 Mm)
Polar radius 1.079 58 PJ (72.243 Mm)
Mean circumference 475.436 Mm
Equatorial circumference 486.194 Mm
Polar circumference 453.919 Mm
Surface area 1.170 31 SJ (71 876 Mm²)
Volume 1.266 05 VJ (1.812 0 × 106 Mm³)
Flattening 0.067 89 (1:14.731)
Angular diameter 54.426 μas
Mass 0.992 482 MJ
Reciprocal mass
relative to star
Density 1.040 g/cm³
Gravitational influence
Surface gravity 2.239 g (21.95 m/s²)
Weight on Alpheus
(150 lb on Earth)
336 lb
Standard gravitational parameter 1.257 × 108 km³/s²
Escape velocity 57.65 km/s
Hill radius 92.189 LD (35.437 4 Gm)
Roche limit
(3 g/cm3 satellite)
0.174 23 LD (66.973 Mm)
Stationary orbit 0.416 73 LD (160.191 Mm)
Stationary velocity 23.043 km/s (5.179 LD/d)
Rotation characteristics
Rotation period 17.9944 h (0.749 768 d)
Rotation velocity 7.505 km/s (20.01°/h)
Direction of rotation
relative to orbit
Axial tilt 23.088°
Longitude of vernal equinox 252.338°
North pole right ascension 19h 38m 42s (294.673°)
North pole declination +16° 32' 31" (+16.542°)
North polar constellation Sagitta
North polar caelregio Tarandus
South pole right ascension 07h 38m 42s (114.673°)
South pole declination −16° 32' 31" (−16.542°)
South polar constellation Puppis
South polar caelregio Malus
Thermal characteristics
Surface temperature 150 K (−123°C, −189°F, 271°R)
Mean irradiance 47.7 W/m² (0.034 9 I)
Irradiance at periastron 52.2 W/m² (0.038 1 I)
Irradiance at apastron 43.7 W/m² (0.032 0 I)
Albedo 0.414 (bond), 0.524 (geom.)
Scale height 28.91 km
Surface density 0.227 g/m³
Molar mass 2.20 g/mol
Composition 90.987% hydrogen (H2)
9.007% helium (He)
24.7 ppm ammonia (NH3)
17.7 ppm methane (CH4)
7.25 ppm water (H2O)
4.01 ppm ethane (C2H6)
433 ppb hydrogen deuteride (HD)
18.8 ppb neon (Ne)
7.72 ppt benzene (C6H6)
3.08 ppt phosphine (PH3)
Ammonia (NH3)
Water (H2O)
Ammonium hydrosulfide (NH4SH)
Dipole strength 328 μT (3.28 G)
Magnetic moment 2.57 × 1020 T•m³
Dipole tilt 8.73°
Satellite system
Number of moons 159
Number of rings 20

Alpheus (Gliese 651 b, P170) is a planet which orbits the yellow G-type main sequence star Gliese 651 (usually referred as HD 154345). The star is slightly smaller, cooler and thus dimmer than our Sun. It is approximately 61 light-years or 19 parsecs from Earth towards the constellation Hercules in the caelregio Tarandus.

Alpheus orbits at a distance of 4.2 AU (1 AU closer to the star than Jupiter's distance from our Sun) in an oval path (e=0.26). Alpheus is very slightly less massive and slightly bigger than Jupiter.

Alpheus is named after the river god in Greek mythology.

Discovery and chronology Edit

Alpheus was discovered on March 12, 2006 by a team of astronomers led by Jason Wright. The team used the spectrometer mounted on the telescope in Keck Observatory and found that this star wobble caused by an orbiting planet. However, Alpheus wasn't confirmed until May 27, 2007. The planet was discovered to have minimum mass 2.03 MJ and orbited at a distance of 9.21 AU (but it has since been revised to 0.963 MJ and 4.19 AU respectively).

Alpheus is the 163rd exoplanet discovered overall, 137th exoplanet discovered since 2000, and 6th exoplanet discovered in 2006. Alpheus is also the 4th exoplanet discovered in the constellation Hercules (1st in 2006) and 15th in the caelregio Tarandus (2nd in 2006). Alpheus is the first and only planet discovered in the Gliese 651 system, receiving the designations Gliese 651 b (a is not used because the parent star uses this letter to reduce confusion) and Gliese 651 P1. Note that the chronology does not include speculative brown dwarfs (objects with minimum masses below 13 MJ but with speculative true masses above 13 MJ).

Orbit and rotation Edit

Orbit Edit

Alpheus takes nearly a decade to orbit around the star at an average distance of 652 gigameters (4.36 AU). Light from its parent star would take more than four times longer to reach Alpheus than light from our Sun to reach Earth at more than 36 minutes. Unlike the gas giants in our solar system, Alpheus orbits in an oval path with an eccentricity of 0.26. However, previous estimate put an eccentricity of 0.04. The planet moves at an average velocity of 13.3 km/s (2.79 AU/yr). The plane of its orbit is edge-on with an inclination of 87.5°.

This planet is in the prototype orbit from which this orbit class originated from, called Alphian orbit (A orbit), which ranges from 2.5 to 5 AU. But due to its eccentricity, Alpheus would spend some time outside of the Alphian orbit range, in this case farther out to 5.5 AU in the Jovian orbit (J orbit) territory.

Parent star observation and irradiance Edit

Viewed from Alpheus, the parent star would appear to be just 3% as bright as the Sun seen from Earth or 32 times fainter. The parent star has a magnitude −22.98 compared to −26.74 for the Sun's magnitude viewed from Earth. Viewed from Alpheus, the parent star would have an angular diameter of 6.4', which is 15 the angular diameter of the full moon we see every month.

Alpheus receives 3.5% of the Earth's irradiance from the sun because it orbits over 413 times farther away from the star whose luminosity is 35 that of our Sun.

Rotation Edit

Alpheus takes 18 hours to rotate once on its axis, which is three quarters of an Earth day. A year on Alpheus is 4454 days compared to 366 days on Earth. The planet tilts 23.1° to the plane of its orbit, very similar to the 23.4° tilt of Earth. The planet's north pole points to the constellation Sagitta (in Tarandus), while the south pole points to the constellation Puppis (in Malus).

Structure and composition Edit

Mass and size Edit

Alpheus' mass is almost identical to that of Jupiter, the most massive planet in our solar system. It is classified as mid-Jupiter in the planetary mass classification scheme. It is slightly larger than Jupiter, meaning that Alpheus is a bit less dense and weaker gravity than Jupiter. These correspond that Alpheus' escape velocity is slightly lower than Jupiter's. It is a gas giant with no solid surface with mean density slightly greater than water.

Like all other planets, Alpheus is not a perfect sphere like a soccer ball, but an ellipsoid. Its equatorial diameter is 6384 miles wider than its polar diameter. Its flattening (ellipticity) is 0.0679, similar to Jupiter's.

Gravitational influence Edit

Alpheus' surface gravity is well over twice as strong as Earth's. If you weigh 150 pounds on Earth, you would weigh 336 pounds on Alpheus. So the average mass of human on Alpheus would be just as heavy as an obese person!

Lying at 0.174 lunar distances or 89% the radius of the planet is the maximum distance where a moon can shred to pieces caused by the planet's tidal forces, called its roche limit. However, this doesn't apply to all kinds of moons. The roche limit depends on the moon's density, a value of 3 g/cm³ is set as the standard because I believe that the average moon density is around that value. Denser moons would have to orbit closer to its parent planet in order for tidal forces to shred to pieces, and vice versa. The hill radius, which is the radius of the hill sphere within where moon's orbits are stable, is 92 LD or 468 <abbr title="Rp">planetary radii</abbr>. If moons are orbiting beyond that sphere, gravitational influences of the grandparent star would cause moons to have unstable orbits around the planet until it begins to orbit the star. The further away from the parent planet's hill sphere, the sooner it takes for those objects to start orbiting the star. The stationary orbit, where the satellite's orbital period is identical to rotation period of the planet, is 0.417 LD or 2.07 <abbr title="Rp">planetary radii</abbr>. The stationary velocity, the orbital velocity at stationary orbit, is 23.0 km/s or 14.3 mi/s. Since the planet takes 18 hours to rotate, then a moon would also take 18 hours to orbit the planet at stationary orbit.

Interior Edit

Below Alpheus' outer envelope (atmosphere), the weight of all the gases pressing down produce a tremendous pressure. That pressure allow hydrogen and helium to condense in the upper mantle despite the higher temperatures deeper down. In the middle mantle lies liquid metallic hydrogen where hydrogen can conduct electricity under even greater pressure heated beyond its critical point. In the middle mantle, the temperature is 12,100 K (11,800°C, 21,300°F) and the pressure 710 MPa. In the lower mantle, there is narrow layer of solid metallic hydrogen at a pressure of 1.8 GPa and temperature 27,900 K (27,700°C, 49,800°F). At the center lies a core of rock and metal with a mass 13 Earth masses, roughly 4.3% the total mass of the planet. The temperature of the core is estimated to be 35,300 K (35,000°C, 63,000°F) and an estimated pressure 3.4 GPa.

Atmosphere Edit

Alpheus' atmosphere composes about 91% hydrogen and 9% helium. However, Alpheus also contains trace amounts of other gases, including ammonia, methane, and water vapor, having concentrations of 25, 18, and 7 parts per million respectively. The atmosphere also contains 4 ppm ethane, which is about 2½ times more concentrated than methane on Earth. There are also tiny amounts of gases with concentrations of only in parts per billion or even in parts per trillion, such as hydrogen deuteride (a molecule composing of two isotopes of hydrogen), neon, benzene, and phosphine.

Alpheus contains banded clouds of ammonia and water and this planet would appear as orange and white stripes from space. The ammonia clouds are in the cooler, upper deck and water clouds in the warmer, lower deck. The planet's temperature is 150 K (−123°C, −189°F). This planet radiates twice the amount of energy than it receives from the parent star. Like Jupiter, there are hundreds of jet streams and zonal jets, which can produce violent long-lasting storms and high winds.

Magnetic field Edit

This planet has a strong magnetic field, about 3.28 gauss, which is three quarters the magnetic field strength of Jupiter.

That powerful magnetic field is produced by the movements of metallic hydrogen in its interior caused by the planet's rotation. This mechanism is well known as dynamo effect. The magnetic field blocks most of stellar and cosmic radiation from reaching the planet, but occasionally it can produce beautiful, vivid aurorae when the stellar radiation got caught in the magnetic field lines and move towards their poles where it interact with the planet's upper atmosphere (ionosphere).

Moons and rings Edit

Alpheus has 159 moons and it has 20 dusty rings. The largest moon has mass 4.6 Lunar masses (0.056 Earth masses) and has diameter 1.556 Lunar diameters (3,359 miles, 5,406 kilometers), which is the same mass as Mercury and slightly larger. There are two other moons that are bigger than our Moon, five with diameters between 1000 miles and the diameter of our Moon, and 29 have diameters between 100 and 1000 miles. All the rest (122) are less than 100 miles in diameter, the majority of which (79) are less than 10 miles.

Future studies Edit

The probability that Alpheus will transit Gliese 651 can be a slim 0.31% chance, but it is speculated that Alpheus will transit. Finding this transiting planet is very challenging because it can only transit the star once every nine years, so the star has to be monitored continuously very likely for years until transit is found. Even one transit would be enough to constrain the size and inclination of this planet. The derivative parameters, including density and surface gravity, can then be calculated using the radius constrained from transit and true mass calculated by inclination. Using the calculated density, astronomers can model the interior of this planet.

If Alpheus does not transit, then this planet can still be studied using different methods, like astrometry or direct imaging. The planet can be studied using astrometry using Gaia, James Webb Space Telescope (JWST), Space Interoferometry Mission (SIM), or even the current Hubble Space Telescope (HST) guidance sensor. The direct imaging can see what the planet may really look like.

Astronomers may eventually use astroseismology to study the interior, including the extent, features and compositions by layers. Using the spectrometer mounted on the JWST, the atmosphere can be studied, including temperatures, chemical makeup, and features. Using the same method, the rotation rate can be constrained using Doppler shifts, which in turn rotation period can then be calculated using the equatorial circumference.

In orbit around the planet, moons can be detected using the transit across the planet, detecting the wobble of the planet, or even direct imaging. Rings can also be detected using just two methods: transit or direct imaging.

Alpheus can further be studied using the JWST's successor: ATLAST, due to launch between 2025–35.